This phase II, multi-national, double-blind, placebo-controlled, parallel group study will assess the safety and potential of CD571 in treating patients with moderate to severe Crohn's disease following an IV dose of 20 or 10 mg/kg of CDP571 followed by 8- or 12- weekly dosing with 10 mg/kg CDP571 over 24 weeks. Subjects will be monitored throughout for safety. Primary efficacy endpoint is the percentage of patients who show a response to treatment (a decrease in CDAI >/= 70 points) at 2 weeks.